Typically, in an Ethernet-over-OTN (EoO) network performing virtual concatenation (VCAT) transmission (Ethernet is a registered trademark and OTN stands for optical transport network), line quality evaluation information regarding a transmission path is measured by transmitting a test signal such as a pseudo-random bit sequence (PRBS) signal, as stipulated in the ITU-T G.709 (OTN) standard. Moreover, although not stipulated in the current ITU-T standard, there is a demand that the technology of measuring line quality evaluation information using a PRBS signal as a test signal be applied to an Ethernet-over-SDH/SONET (EoS) network or an Ethernet-over-PDH (EoP) network in an identical manner to that of an EoO network (SDH stands for synchronous digital hierarchy, SONET stands for synchronous optical networking, and PDH stands for plesiochronous digital hierarchy).
To perform full transmission of a 1 Gigabit Ethernet (GbE) signal in an EoS network as illustrated in FIG. 7, seven VC4 signals (VCAT members) are virtually concatenated as a VC4-7V signal that is then transmitted over an SDH network. That makes it possible to effectively use the transmission band. In comparison, when VCAT transmission is not performed, the transmission is performed with a VC4-16C signal thereby resulting in a waste of 50% or more of the transmission band.
Meanwhile, it is not necessary that all seven VCAT members (VC4 signals) are transmitted over the same transmission path. That is, depending on the usage condition of the transmission line, the seven VC4 signals are sometimes divided into, for example, a group of four VC4 signals and a group of three VC4 signals and each group of VC4 signals is transmitted over a separate transmission path as illustrated in FIG. 7.
For such a case, there is a demand that an EoS apparatus (VCAT transmission apparatus) in a VCAT transmission system be equipped with a line quality evaluation feature using a PRBS signal (PRBS transmission feature, PRBS monitoring feature) to evaluate the line quality.
In practice, line quality evaluation using a PRBS signal can be performed with either an EoS processing unit illustrated in FIG. 8 that enables line quality evaluation of the entire VCAT signal (VC4-7V signal in the abovementioned example) or an EoS processing unit illustrated in FIG. 9 that enables elementary line quality evaluation of each VC path on an individual basis without giving consideration to the VCAT signal.
In the EoS processing unit illustrated in FIG. 8, line quality evaluation of the entire VCAT signal (VC4-7V signal in the abovementioned example) is performed by receiving a PRBS signal that is inserted in serial with respect to the entire VCAT signal and calculating an overall error rate for the entire VCAT signal.
In comparison, to perform elementary line quality evaluation of each VC path on an individual basis without giving consideration to the VCAT signal in the EoS processing unit illustrated in FIG. 9, a selector is disposed corresponding to each of seven VC signal processing units #1 to #7. Then, a PRBS generation/monitoring unit is selected for each VCAT member (7 times for each VC4 signal and 21 times for each VC3 signal) to individually evaluate the line quality of each VCAT member. Alternatively, a plurality of PRBS generation/monitoring units (7 for a VC4 signal and 21 for a VC3 signal) can be disposed for individual evaluation of line quality (see Japanese Laid-open Patent Publication No. 2001-268038 for further description of the conventional technique).
Meanwhile, in the technology of line quality evaluation of the entire VCAT signal, the line quality of individual VCAT members cannot be verified in consideration of division of the transmission path in VCAT transmission. That makes it difficult to independently evaluate the line quality of each transmission path.
In a conventional EoS apparatus, a VC-nC signal is de-multiplexed into a VC signal by performing byte interleaving and is transmitted in parallel as a VC-nV signal to individual VCAT members. Thus, in a network in which, as illustrated in FIG. 7, transmission is performed over two transmission paths by dividing the VCAT signal into a group of four VC4 signals and a group of three VC4 signals, the signal sequence of the PRBS signal gets disturbed (see FIG. 10).
For that reason, when the transmission path is divided, it is not possible to obtain the error rate of each divided transmission path on an individual basis. As a result, the line quality of each VCAT member cannot be verified individually at the same time. That makes it difficult to independently evaluate the line quality of each transmission path.
On the other hand, in the technology of simple line quality evaluation of each VC path on an individual basis without giving consideration to the VCAT signal, it is necessary to dispose a selector corresponding to each of the VC signal processing units #1 to #7 or dispose a plurality of PRBS generation/monitoring units. However, that is not an efficient way of performing line quality evaluation.